1. Field of invention
The invention relates generally to communication networks and in particular to a method and system for providing mesh protection services in a communications network.
2. Description of Related Art
In conventional communications networks, protection techniques are used to provide bandwidth for one or more working paths in the event the working path fails. A variety of protection topologies exist and include assigning a single protection path for each working path (referred to as 1:1 protection) or multiple working paths sharing a protection path (referred to as 1:N protection). Protection topologies also vary depending on the network topology and may be implemented in linear, ring or mesh configurations. Protection mechanisms may be implemented at the link level such as automatic protection switching (APS) or at higher levels such as ring level bidirectional line switched ring (BLSR).
FIG. 1 depicts a portion of an exemplary conventional communications network. The network includes network elements 10, 12, 14 and 16 coupled together by a number of working links W1-W12. Each working link can be implemented through the use of a unidirectional link carrying signals in one of two directions, often referred to as “east” or “west”. For illustrative purposes only, network element 10 will be considered the source (e.g., originating node) for network traffic directed to network element 16 (e.g., terminating node) through network elements 12 and 14.
In the event of a failure in a working line, due to the nature of uni-directional links, the system of FIG. 1 has difficulties in restoring network traffic. The failure may be a hard failure (e.g., loss of signal) or a soft failure (e.g., degradation of signal). For example, if working link W5 experiences a failure 13 (e.g., a hard failure), network element 14, which in this example was receiving a signal from network element 12, will stop receiving the signal from network element 12. This loss of signal with network element 12 enables network element 14 to detect the failure on the link with network element 12. Upon identifying a failure on working link W5, network element 14 will generate a release message signal 15, which is transmitted to network element 16 to inform network element 16 of the failure. Network element 14, however, does not generate its own release message signal to network element 12. The reason for this limitation is that the release message signal is a path-level message that relies upon bi-directional communication for providing reliable delivery. Because working link W5 has failed, bidirectional communication between network elements 12 and 14 is not available. Thus, no signaling release message signal is generated from network element 14 to network element 12.
Since network element 12 does not receive a release message signal from network element 14 and the unidirectional working link W6 with network element 14 still provides network element 12 with a signal, network element 12 is not aware of the link failure on working link W5. Therefore, network element 12 will either receive the release message signal from network element 10 after it traverses across the network, or alternatively for other reasons may never receive the notification of the link failure.